The invention relates to submersible articles and parts for submersible articles, including submersible vehicles, and a method of manufacture of submersible articles and parts therefor.
Many kinds of submersible article are known. These include propulsion units for divers, which may be hand-held or configured to be strapped to a diver""s body, underwater exploration vehicles which may be manned or remote-controlled, and mooring buoys for marine tankers. Articles of this type are required to function satisfactorily in an underwater environment, and hence need to be constructed in such a way as to, and from materials which, permit this. For example, the materials and construction need to give sufficient buoyancy while at the same time being strong enough to withstand hydrostatic pressures at the depth of water for which the article is designed. Materials need to be waterproof, and preferably not overly heavy, both with regard to achieving a desired buoyancy and also for ease of carrying if the article is designed to be portable. Also, the construction needs to be suitable for housing components of the article such as motors, batteries, propellers and the like.
The use of various plastics materials in submersible and other aquatic articles has been proposed with the aim of meeting the above and other objectives. Syntactic epoxy foam, which is a foam containing hollow glass microspheres, is suitable for remote-controlled submersible vehicles designed for deep water use, as it can withstand depths in excess of 2000 m.
U.S. Pat. No. 5,634,423 discloses a personal submersible marine vehicle having a hull in the form of a hollow shell made from polyethylene by rotation molding. The shell houses a battery and control units (and weights to adjust the buoyancy) which are electrically insulated from each other by high density polyurethane foam which is pumped into the shell after the battery and units are installed to fill the remaining space in the shell. This gives a watertight hull which is neutrally buoyant and protected on the outside by the rigid polyethylene. However, once embedded in the polyurethane, the battery and units are difficult to replace if they become faulty. It is also necessary to achieve a good bond between the two types of plastic to maintain the structure of the hull and to ensure it is watertight, as an opening in the polyethylene shell is needed through which the polyurethane is pumped.
U.S. Pat. No. 6,224,706 discloses a surfboard or sailboard made from expanded polystyrene which is strengthened by being encapsulated in a sheet of thermoplastic material. Encapsulation is achieved by laying heated thermoplastic material over the polystyrene board, and drawing air through the permeable polystyrene so that the thermoplastic conforms to the shape of the board. This gives a light yet strong structure. However, the use of sheet material to form an outside shell by effectively wrapping the sheet around the polystyrene is disadvantageous as regards achieving a visually pleasing exterior finish, which is desirable in articles designed for leisure purposes.
According to one aspect of the invention there is provided a submersible article, comprising: a buoyancy body formed from polyurethane foam of a first density; and a protective skin formed from polyurethane foam of a second density higher than the first density, wherein the protective skin is overmolded onto the buoyancy body.
The lower density of the inner, buoyancy body gives a desired degree of buoyancy, while the higher density protective skin forms a tough and strong outer shell which protects the inner body and provides impact resistance. Use of high density foam alone to mold the entire article would give desirable toughness and strength, but the article would be too heavy to be sufficiently buoyant for submersible use. This is overcome by the proposed two-layer structure, in which the lower density foam used for the inner part is light enough to provide buoyancy. The molded aspect of the outer layer permits a good surface finish to be given to the article. Also, polyurethane has good adhesive properties, so that the use of it for both layers means that the inner and outer polyurethane parts bond strongly together to give a robust article.
The density of the inner body may be between 100 kg/m3 and 600 kg/m3, and the density of the outer skin may be between 300 kg/m3 and 1200 kg/m3. Other values outside this range or subranges within this range may also be used. An inner body density of 300 kg/m3 is high enough to allow the article to withstand the hydrostatic pressure of water at a depth of about 150 m.
If desired, the outer protective skin may comprise reinforcing material which increases the stiffness of the outer protective skin. A semi-flexible polyurethane can be used for the outer protective skin, but this may not be satisfactorily stiff for particular uses of the article. For example, the reinforcing material may be distributed unevenly throughout the outer protective skin so as to impart a stiffness which varies over the outer protective skin in a preselected manner. In this way, a greater stiffness can be given to areas such as carrying handles.
Preferably, the reinforcing material comprises glass fiber. This is incorporated suitably throughout the outer protective skin during the molding of the polyurethane, so as to impart the desired stiffness where required.
According to another aspect of the invention there is provided a method of manufacturing a submersible article, comprising: providing a buoyancy body comprising polyurethane foam of a first density; placing the buoyancy body in a mold; and overmolding a protective skin onto the buoyancy body using polyurethane foam of a second density higher than the first density.
In an embodiment, the method comprises: providing a first mold and a second mold; using the first mold to form, from polyurethane foam having a first density, an inner body with an outer surface; removing the inner body from the first mold; placing the inner body in the second mold; and using the second mold to form, from molded polyurethane foam having a second density which is higher than the first density, an outer protective skin which adheres to and at least partially covers the outer surface of the inner body.
This method can provide a straightforward way of manufacturing an submersible article having the desired qualities of buoyancy and strength. Use of polyurethane foam for both inner and outer parts means that similar molding processes can be used to form each part. This simplifies manufacturing and reduces costs compared to the production of previously proposed articles having parts formed from different materials by different processes.
The method permits a wide variety of shapes of article to be produced. This contrasts with prior art methods, in which molding is only used for one part, so that either the outer shell or the inner buoyant component must conform to the shape of the other part. Also, the formation of the outer protective skin by overmolding the inner block gives a good exterior finish, good contact and a strong bond between the parts, and the outer protective skin can be configured to cover as much or as little of the inner block as desired, by use of an appropriate mold.
The method may additionally comprise: arranging reinforcing material over at least part of the buoyancy body before the overmolding so that the reinforcing material embeds in the polyurethane foam of the protective skin during the overmolding. The reinforcing material can be used to increase the stiffness of the outer protective skin. Preferably, the reinforcing material is fastened to the inner body to inhibit migration of the reinforcing material through the outer protective skin. Fastening can stop the reinforcing material from moving through to the outside of the outer protective skin during molding, which can have a detrimental effect on the exterior finish of the article. Glass fiber may be used as the reinforcing material.
In a preferred embodiment, the reinforcing material is distributed unevenly over the outer surface, so as to impart a stiffness which varies over the outer protective skin in a preselected manner. Hence the outer protective skin can be stiffened locally if required, for example, around carrying handles and the like. This is made possible by the application of the reinforcing material over the inner body; if the reinforcing material were added directly to the higher density polyurethane before molding, it would not be possible to control its distribution.
Advantageously, the method further comprises, abrading at least part of the outer surface of the inner body prior to the overmolding. Polyurethane has good natural adhesive properties, so that the use of it for both parts of the article gives a good bond between the parts; however, the strength of the bond is improved by abrading the inner body to provide a key for the outer protective skin. Also, abrasion may be used to remove any release agent used in the manufacture of the inner body.
The mold used for the overmolding may be provided with at least one locator operable to position the inner body within the second mold in a preselected location with respect to the second mold. This gives greater flexibility in the shape of the outer protective skin, so that it can be more than just a skin conforming to the shape of the inner body. Also, the provision of fasteners allows the outer protective skin to be formed over the whole of the inner body in a single molding step, as the inner body can be held away from the sides of the second mold at all points.
Another aspect of the present invention is directed to a submersible vehicle having a hull comprising: a buoyancy body formed from polyurethane foam of a first density; and a protective skin formed from polyurethane foam of a second density higher than the first density, wherein the protective skin is overmolded onto the buoyancy body.